Resin foams such as polyolefin resin foams, polyurethane resin foams and the like have been used in a wide variety of industrial fields. This is because they possess various desirable features which include, but are not limited to, lightweight, thermal insulation, processability and shock absorption. Although the resin foams are typically lightweight, they are very difficult to recycle due to their bulky size. This is especially true for crosslinked resin foams. Furthermore, since the resin foams persist semi-permanently even after being buried in the ground, sites for incineration or landfill disposals of the wasted foams are being rapidly depleted, often resulting in serious environment pollution and landscape waste.
In order to address and alleviate such situation, biodegradable resins which are capable of being decomposed in the nature by microorganisms have been investigated and developed. Typically, they are commercially available as films or fibers. Furthermore, extruded foams of biodegradable resins have also been developed. One example which is currently known is non-crosslinked foams comprising aliphatic polyester resin. However, it was extremely unfeasible, if not virtually impossible, to form high molecular weight aliphatic polyester resins due to undesirable side reactions, such as hydrolysis which is of ten times caused by water generated during polycondensation. As a result, the melt viscosity during extrusion foaming is not sufficient to maintain optimal or even adequate cell structure. As such, foams having good cell structure and surface appearance cannot be obtained.
As an approach to resolve this problem, Japanese Patent No. 2655796 suggests a method of crosslinking resin using an ionized radiation. However, the method has drawbacks and deficiencies in that radiation cannot reach the inner portion of the subject to be radiated as it is over 1 mm thick and the cells in the inner portion of the subject upon foaming are coarse and uneven. Also, crosslinking by radiation should be carried out under N2 atmosphere in order to prevent deterioration of resin. Thus, it is very difficult to prepare foams having various thickness and sufficient mechanical properties through the implementation of this method.
In addition, Japanese Laid Open Patent Publication No. (Hei)11-279311 discloses foams comprising lactone resin. Since degradation of lactone resin also takes place simultaneously with crosslinking when it is subjected to treatment by radiation, the melt viscosity during foaming is not sufficient to maintain adequate cell structure. Thus, it is extremely difficult to obtain foams with good surface appearance. In other words, irradiation at around room temperature typically requires the exposure of as high as 200 kGy. To address this problem, it is described that after melting the lactone resin at least to its melting point, irradiation is preferably carried out in its melt phase before crystallization. Thus, at low exposure, foams having high crosslinking degree (gel fraction) cannot be readily prepared.
Japanese Laid Open Patent Publication No. (Hei) 10-254511 discloses a method for preparing the foams generally comprising the steps of: (1) filling a mold with a resin composition consisting of starch, polyethylene resin, a thermal decomposable blowing agent and an organic peroxide; (2) decomposing the blowing agent and organic peroxide by exposure to pressure and heating; and (3) removing the pressure. However, since the decomposition temperature of an organic peroxide is similar to that of a blowing agent, the cell size of the obtained foams is uneven. Also, foams which have excellent appearance and which are in a continuous sheet form cannot be prepared by this method.
Japanese Patent No. 3229978 discloses a method for preparing aliphatic polyester resin foam particles having a gel fraction of at least 5%. Such method is typically performed by using an organic peroxide and a compound having at least two unsaturated bonds (i.e., a crosslinking promoter). In order to crosslink the base resin particles, the method essentially resorts to the steps of: (1) dispersing the particles into a dispersing medium such as water, ethylene glycol and methanol; (2) heating the dispersion after adding an organic peroxide and, if necessary, a crosslinking promoter. Thus, the foam particles prepared by this method may be hydrolyzed which may result in undesirable and non-optimal foam particles. Furthermore, when these particles are subjected to a secondary process of forming them into a desired shape, a problem typically arises in that such particles may break out at their laminated sides. Thus, formation of a complicated and/or sophisticated shape is rarely possible when utilizing this method. Consequently, this method is incapable of making crosslinked foams in a continuous sheet form.
Japanese Laid Open Patent Publication No. (Sho) 46-38716 discloses a method for continuously producing polypropylene foams comprising random propylene/ethylene copolymer. This reference also discloses that the use of a crosslinking promoter enables a crosslinking reaction to be efficiently and smoothly performed.
Also, Japanese Laid Open Patent Publication No. (Sho)60-28852 discloses a method of adding a crosslinking promoter to a mixture of random propylene/ethylene copolymer and polyethylene, and further crosslinking and foaming the product. However, this method has certain drawbacks and deficiencies. More specifically, because the polypropylene foams obtained by this method are crosslinked, they cannot be recycled and also they are not biodegradable. Thus, it is extremely difficult to dispose such foams. In addition, combustion calories for incinerating the foams are so high that they have negative influences and impacts on the global environment.
In summary, there is currently no method available for producing a continuous foam sheet which has a sufficient crosslinking degree and which is formed from a biodegradable resin. In view of such shortcoming, the present invention has been formulated after an extensive research to produce a crosslinked biodegradable resin continuous foam sheet, wherein the sheet has all the desired processability, mechanical properties, thermal properties and recyclability. More particularly, the present invention is found on a premise/that the crosslinked biodegradable resin continuous foam sheet can be prepared by using a biodegradable resin and a crosslinking promoter, hence achieving the primary objective of the present invention.